To control electronically commutated electric machines, electric control variables are applied to the stator windings of the machine, as a function of the angle position of the rotor, for the purpose of driving the machine. The rotor position is generally detected with the aid of a rotor position encoder and supplied to a control unit for the purpose of generating the control signals needed to commutate the electric machine. Rotor position encoders supply either an analog electrical variable dependent on the position of the rotor, e.g., a voltage, signal pulses or a digitized indication of the absolute rotor position. Rotor position encoders are known from the prior art, in which a signal encoder (magnetic target), which is non-rotatably supported on the rotor, is read out with the aid of a magnetic field sensor non-rotatably supported on the stator.
DE 10 2009 001 353 A1 shows an electric machine, which includes a rotor having a rotor hub, a stator situated in a stator housing, a cover which is connected to the stator housing and extends to the inner diameter of the rotor hub and via which the rotor is supported with the aid of a rotor bearing. The electric machine has a rotor position sensor for detecting the rotational position of the rotor with respect to the magnetic field of the stator. The rotor position sensor is situated on the cover in the proximity of the rotor bearing in such a way that the rotor hub or a component which is non-rotatably connected to the rotor hub is used as the encoder track.
WO 2011/092320 A1 deals with a sensor unit for an electric machine, which includes a position sensor for providing an indication of the rotor position, a motor characteristic map unit having a motor characteristic map device for the purpose of modifying the indication of the rotor position according to a predefined motor characteristic map, as a function of the operating point, and an interface unit for the purpose of providing the modified indication of the motor position to a control unit
For the zero position of the signal encoder of the rotor position encoder to match the defined zero position of the rotor, a corresponding angular offset must be stored in the control unit. The angular offset is generally ascertained for a concrete installation situation and stored in the control unit. To ascertain the latter, measuring methods are known which must be implemented in the control software of the control unit and which require certain operating states of the electric machine or may be carried out only in a repair shop. In known methods, the motor is driven to a minimum speed, which, in turn, makes it possible to compare the rotor position encoder sensor signal with the counter-EMF over a sufficiently long period of time in the undriven state and thus to determine the phase shift between the sensor signal and the real rotor position. If the position of the signal encoder (magnetic target) detected by the rotor position sensor with respect to the rotor changes, for example during replacement in the event of servicing, the stored offset value is no longer correct and must consequently be remeasured.
DE 10 2008 042 829 A1 describes a method and a device for compensating for the offset of a rotor position encoder of an electric machine, in which the points of intersection of the phase voltage signals of the electric machine are ascertained using comparators. The signals provided by the comparators are converted by a logic unit into a single-channel signal, which is supplied to a microcomputer. In the method, the rotor of the electric machine is accelerated to a predefined rotational speed with the aid of an injected rotating field. As soon as the rotor reaches the predefined rotational speed, the electric machine is switched to the free-wheeling state, a sinusoidal course of the phase voltages which are measurable at the machine terminals setting in. The offset of the rotor position encoder is ascertainable by comparing the phase voltages and the pulse sequences provided by the rotor position encoder.
DE 102 53 388 B4 shows a method for adjusting a sensor device for determining the rotational position of a rotor of an electronically commutated motor, in which the sensor device is mounted in a certain position relative to the rotor. The increments generated by the sensor device during one rotation of the rotor are detected. The motor is driven, and the voltages induced by the motor are detected, the angular position of the rotor and a sought commutation angle being derived from the induced voltages. The detected angular position is correlated with the increments of the sensor device. The correlation of the angular position and the increments of the sensor device are stored. One disadvantage of the described method is that it is also unable to be carried out during operation but requires a special adjusting device and thus makes it necessary to seek out a repair shop.
DE 10 2012 204 147 A1 shows a method for controlling an electronically commutated electric motor having multiple phases in a drive system of a motor vehicle, which includes a rotor and an absolutely measuring rotor position sensor, which monitors the rotation angle of the rotor, and an electronic system for commutating the electric motor on the basis of detected data of the rotation angle of the rotor position sensor. A plausibility check takes place regularly over the service life of the electric motor by setting a predefined actual position of the rotor position while the electric motor is stopped. For this purpose, phases u, v, w are simultaneously applied to current coils situated in a stationary manner in the stator, so that a stationary magnetic field sets in, toward which the pole pair is oriented and thereby positions the rotor exactly in the predefined actual position. This actual position is compared with a test position detected by the rotor position sensor. Taking into account an angular offset, which may have already been ascertained in the factory, the actual position and the test position are compared with each other and checked for the exceeding of a limiting value. The plausibility check may be carried out, for example, when the motor vehicle is started.
A method is known from DE 10 2011 105 502 A1 for compensating for a phase shift between a rotor position sensor and a rotor position of an electronically commutated motor, which may be carried out both during startup and during operation of the motor. The position of the rotor is measured with the aid of an absolute value rotor position sensor, which is compared with a motor parameter which characterizes the expected position of the rotor. The phase shift which occurs, for example, during assembly of the motor and the rotor position sensor, may be automatically corrected hereby, even during operation. During the operation of the motor, the motor is activated with the aid of a block commutation, in which one phase is always current-free. The phase voltage at the current-free phase is measured. At the point where the zero crossing of the phase voltage is situated, the latter is aligned with the position which the absolute value rotor position sensor measures. A piece of information about the position of the rotor is established via the phase voltage. To determine whether a phase shift is present, the position actually determined is compared with an expected value of the rotor position. The described method requires the provision of corresponding software functions in the control unit for ascertaining the angular offset.